There are known coin-shaped storage cells such as electric double layer capacitors and button cells. FIG. 9 is a cross-sectional view showing a construction of a conventional coin-shaped electric double layer capacitor. Referring to FIG. 9, the capacitor is constructed in such a manner that a polarizable electrode 88 at a cathode (positive) side, which is an activated carbon electrode, and a polarizable electrode 87 at an anode (negative) side, which is an activated carbon electrode, are opposed to each other via an insulating separator 91, thereby forming an electrode pair. A positive electrode collector 90 and a negative electrode collector 89 are provided on the cathode-side polarizable electrode 88 and the anode-side polarizable electrode 87, respectively.
The capacitor in FIG. 9 is produced by impregnating an electrolytic solution 92 in the polarizable electrode pair 87, 88, and the separator 91, and by housing the electrode pair 87, 88, and the separator 91 in a housing space defined by an upper case 83 serving as an anode terminal and a lower case 81 serving as a cathode terminal. In the production, an electric insulating packing 85 is disposed in a space defined by a bent portion 84 formed at an outer periphery of the upper case 83, and an outer periphery of the lower case 81. Then, the bent portion 84 of the upper case 83 is encased by the packing 85 by curling a tip end 82 of the outer periphery of the lower case 81 inwardly, whereby the housing space for housing the polarizable electrode pair 87, 88 is brought into an airtight sealed state.
In the case of a button cell, a cathode electrode and an anode electrode are housed in a metal case constituted of two case portions by way of a separator. The external construction of the button cell is substantially the same as that of the electric double layer capacitor.
The coin-shaped storage cells are widely used as a main power source and a memory backup source for compact portable devices such as cellular phones. As miniaturization of electronic devices progresses, a demand for the coin-shaped storage cells is yearly increased. In light of the social trend on the electronic devices, it is indispensable to secure long-term high reliability for the coin-shaped storage cells, which is an important element of the electronic devices.
Also, in recent years, as the miniaturization of electronic devices progresses, high integration of electronic parts including the coin-shaped storage cells progresses. A surface mounting by reflow soldering has become a mainstream as a soldering process appropriate for the high integration. The reflow soldering comprises allowing a storage cell mounted on a solder-coated printed board to pass through a furnace of a high-temperature atmosphere of 200° C. or more to solder the storage cell to the printed board. Also, in recent years, as lead-free soldering has been introduced considering environmental issues, reflow soldering with use of tin having a melting point higher than the melting point of lead by about 20° C. has been carried out. In view of this, a higher heat resistance, and a longer lifetime after reflow processing are required for the electronic parts to be mounted on the printed board.
As an approach for securing high reliability of the storage cells, for instance, Japanese Unexamined Patent Publication No. 2003-22935 discloses a technique that the polarizable electrode pair 87, 88 is fixed at a predetermined position by forming a guide portion on the inner bottom surface of the lower case 81 near the polarizable electrode pair, whereby displacement of the electrode pair is prevented.
In addition to the above demand, improvement on liquid-leakage resistance of the coin-shaped storage cells is demanded in order to maintain the quality of the storage cells. Leakage of the electrolytic solution out of the storage cells may cause not only degradation of characteristics of the storage cells but also disorder of peripheral circuits and electronic devices.
In order to improve liquid-leakage resistance of the storage cells, for instance, Japanese Unexamined Patent Publication No. 2000-48780 discloses a technique that the bent portion on the outer periphery of the upper case 83 is made flat, and the width of the flat portion is set in the range of 75 to 150% relative to the thickness of the upper case 83 so as to improve liquid-leakage resistance.
The conventional electric double layer capacitors disclosed in the above publications have a drawback that the vapor pressure of a solvent in an organic electrolytic solution is raised due to a high-temperature condition at the time of lead-free reflow soldering at a soldering temperature of 250° C. or more, which remarkably raises the pressure of the housing space defined by the upper case 81 and the lower case 83, and eventually causes a clearance between the inner bottom surface of the lower case 81 and the bottom surface of the packing 85, thereby causing leakage of the electrolytic solution.